This invention relates generally to display panels, and more particularly to improved liquid crystal display panels including electroless metal plated terminals, and methods for soldering the display panels to a circuit substrate.
Conventional liquid crystal panels which are driven by low voltages and small currents have been used for display devices such as digital watches, electronic calculators and the like. The display area and display capacitance of liquid crystal panels have been increasing since the time when they were first introduced on the market. At that time, developments in design and manufacture of the driving circuits and system of these devices were less advanced than now. As a result, the number of terminals on a panel was generally small. These terminals were electrically connected to the liquid crystal driving circuit through an electrically conductive rubber.
The present use of liquid crystal panels and devices such as, e.g., personal computers and the like, calls for larger display areas and greater display capacitance than earlier utilized or provided. A larger number of terminals is required in these devices. It is difficult to electrically connect the terminals of these liquid crystal panels with the liquid crystal driving circuit through electrically conductive rubber. A flexible circuit substrate including a series of metal contacts, such as copper, has been employed to overcome this difficulty. The following steps electrically connect a flexible circuit substrate with a liquid crystal panel. The substrate is aligned over the panel and the plastic portion of the substrate and glass portion of the panel are fused by thermocompression bonding. Thereby, the terminals of the liquid crystal panel contact the copper layer of the flexible circuit substrate. The drawbacks of this method include contact failure, especially in liquid crystal panels used in display devices in automobiles, where contact can be poor, in view of the vibrations of a moving car.
Conventional processes for providing terminals on liquid crystal panels call for coating a metal layer over the face of the transparent substrate by well known methods, such as sputtering or deposition. A photoresist is selectively applied and then the metal layer, other than that in the terminal regions, would be etched away, prior to the remaining resist being removed to provide the terminals. Upon the completion of manufacture of a panel, a flux agent is applied to the metal coated terminals and then a solder layer is coated thereon prior to electrically connecting the panel to a substrate. It is necessary to apply flux to the terminals prior to applying the solder and then to remove the flux after soldering. These steps, of course add to the expense and time consumed in manufacture of the panels. Moreover, the use of large display area panels calls for soldering a large quantity of terminals. This is conventionally done by dipping the panel substrate in a solder bath. A drawback to this method is that the substrate tends to break as a result of thermoexpansion. To overcome this problem, it is necessary to heat the substrate to approximately 100.degree. C. before dipping. This is another time-consuming and costly step.